Descriptions

In the U.S., the Nuclear Regulatory Commission (NRC) is responsible for setting annual dose limits. For cases of hot particle contamination these limits are set at depths in skin of 7 mg/cm², 300 mg/cm², and 1000 mg/cm². However, at such shallow depths, the lack of charged particle equilibrium (CPE) precludes the use of traditional fluence-to-dose conversion methods.
In this work, an enhanced photon dosimetry model is constructed based on simulations of photon point sources using MCNP5 (Monte Carlo N Particle version 5) transport code. An empirical relationship between KERMA and absorbed dose was established, and used to develop a correction factor, f[subscript CPE], accounting for the lack of charged particle equilibrium (CPE) at shallow depths. This correction factor, in conjunction with traditional point-kernel fluence-to-dose conversion, provides a more accurate prediction of photon dose. The photon model is implemented such that empirical mathematical formulations, rather than look-up tables, drive the estimation of integrated dose over a disk up to 10 cm².
In addition to fCPE, the creation of a secondary off-axis correction (s[subscript C]) was necessary in order to accurately calculate the integrated dose to a disc at shallow depths. When calculating the integrated dose to a disc of 10 cm² at a depth of 0.007 cm for photon energies of 0.662, 0.835, and 1.333 MeV the new model is within 2.0%, 3.7%, and 2.5%, respectively, of the dose calculated by MCNP5. This corresponds to improvements of 134.4%, 165.2%, and 275.5% over VARSKIN 3's results for the same shallow-depth calculations. The photon dosimetry model presented here also incorporates the parameters of energy, attenuation, and dose-averaging area, thereby addressing deficiencies in previous models.